There has been known a developing device which includes a casing configured to accommodate toner, a developing roller disposed to face an opening formed in the casing, and a seal member disposed between end portions of the developing roller and the casing (for example, JP-A-2009-63635).
Incidentally, in the above-described developing device, the developing roller comes into sliding contact with the seal member, whereby a sliding contact portion thereof is heated due to friction. Therefore, when a rotational speed of the developing roller is increased in order to increase a printing speed, the sliding contact portion becomes to have a high temperature and toner melts in the sliding contact portion, and thus there is a concern that toner leakage may occur.